The human eye functions to provide vision by transmitting light through a clear outer portion called the cornea, and focusing the image by way of the lens onto the retina. The quality of the focused image depends upon many factors including the size and shape of the eye, and the transparency of the cornea and lens.
When age or disease causes the lens to become less transparent, vision deteriorates because of the diminished light which can be transmitted to the retina. This deficiency in the lens of the eye is known as a cataract. Ophthalmic surgery is required for treating this condition. More specifically, surgical removal of the deteriorated lens and replacement with an artificial intraocular lens (IOL).
One known technique for removing cataractous lenses is phacoemulsification. During this procedure, a thin phacoemulsification cutting tip is inserted into the diseased lens and vibrated ultrasonically. The vibrating cutting tip liquefies or emulsifies the lens so that the diseased lens may be aspirated out of the eye. Once removed, an artificial lens is inserted therein.
A typical ultrasonic surgical device suitable for ophthalmic procedures includes an ultrasonically driven handpiece, an attached cutting tip, an irrigation sleeve and an electronic control console. The handpiece assembly is attached to the control console by an electric cable and flexible tubing. Through the electric cable, the console varies the power level transmitted by the handpiece to the attached cutting tip and the flexible tubing supplies irrigation fluid to, and draws aspiration fluid from, the eye through the handpiece assembly.
The operative part of the handpiece includes a hollow resonating bar or horn directly attached to a set of piezoelectric crystals. The crystals supply the required ultrasonic vibration needed to drive both the horn and the attached cutting tip during phacoemulsification and are controlled by the console. The crystal/horn assembly is suspended within the hollow body or shell of the handpiece. The handpiece body terminates in a reduced diameter portion or nosecone at the body's distal end. The nosecone accepts the irrigation sleeve. Likewise, the horn bore receives the cutting tip. The cutting tip is adjusted so that the tip projects only a predetermined amount past the open end of the irrigating sleeve.
In use, the ends of the cutting tip and irrigating sleeve are inserted into a small incision of predetermined size in the cornea, sclera, or other location of the eye. The cutting tip is ultrasonically vibrated along its longitudinal axis within the irrigation sleeve by the crystal-driven ultrasonic horn, thereby emulsifying the selected tissue in situ. A hollow bore of the cutting tip communicates with the bore in the horn that in turn communicates with the aspiration line from the handpiece to the console. A reduced pressure or vacuum source in the console draws or aspirates the emulsified tissue from the eye through the open end of the cutting tip, through the cutting tip and horn bores and through the aspiration line, into a collection device. The aspiration of emulsified tissue is aided by a saline flush solution or irrigant that is injected into the surgical site through a small annular gap between the inside surface of the irrigating sleeve and the cutting tip.
Known phacoemulsification systems may even use a surgical cassette to provide a variety of functions for vitreoretinal surgical procedures to assist with effectively managing irrigation and aspiration flows into and out of the surgical site through the surgical device. More specifically, the cassette acts as the interface between surgical instrumentation and the patient and delivers pressurized irrigation and aspiration flows into and out of the eye. A variety of pumping systems have been used in connection with a surgical cassette in fluidics systems for cataract surgery, including positive displacement systems (most commonly, peristaltic pumps) and vacuum based aspiration sources. A peristaltic system uses a series of rollers acting upon an elastomeric conduit to create flow in the direction of rotation, while vacuum based systems employ a vacuum source, typically applied to the aspiration flow through an air-liquid interface.
During surgical procedures, the hollow, resonating tip can become occluded with tissue. In such an instance, vacuum can build in the aspiration line downstream of the occlusion. When the occlusion eventually breaks apart, this pent up vacuum can, depending upon vacuum level and the amount of aspiration path compliance, draw a significant amount of fluid from the eye, thereby increasing the risk of anterior chamber shallowing or collapse. This situation is commonly referred to as occlusion break surge.
To address this concern, surgical consoles are configured with sensors in the aspiration path to allow detection of vacuum level and limiting of vacuum by the system to a predetermined maximum level. While limiting the maximum vacuum level in such a manner may be effective to reduce the potential magnitude of an occlusion break surge, such limitations on the maximum vacuum level can reduce effectiveness of lens removal and increase overall surgical time. In some systems, an audible indication of relative vacuum level and/or vacuum reaching the user preset limit may be provided so that the surgeon can take appropriate precautions.
For example, in some systems, vacuum is commonly relieved upon a command from the surgeon to open a vent valve that connects the aspiration line to a pressure source that is maintained at or above atmospheric pressure. Depending upon the system, this might be the irrigation line, the pump exhaust line or a line connected to atmospheric air (air venting system). However, there are some concerns with known vent valves. First, known vent valves are only configured for simple “on/off” action. For example, pinched tubing valves or elastomer dome type valves may provide satisfactory on/off control of fluid flow but do not exhibit consistent variable flow characteristics. As such, this type of valve has a very sharp surge recovery curve. Moreover, the configuration of dome type valves also may present operational challenges. For example, the operation of the valve is highly dependent upon the elastomer material to obtain a proper seat position, thus consistency of the material is very important. Further, the flow through the valve may also become clogged by debris if the opening formed by the elastomer is small. In addition, such a configuration may undesirably trap air bubbles. Use of these type of valves is also limited in that due to the nature of the on/off flow control limitation, an array of valves are need to support directing fluid flow from one circuit to another.
Alternatively, vacuum may be reduced or relieved by reversal of the pump rotation in positive displacement systems. While it is known to employ a system having bi-directional pump rotation to allow control of pressure/vacuum level based on user input and feedback from a pressure sensor in the aspiration line, such a system requires rapid acceleration and deceleration of the pump head mass. This can limit response time and cause objectionable acoustical noise.
Known cassettes used with consoles also allow the aspiration line to be vented, either to atmosphere or to a liquid so as to reduce or eliminate vacuum surge upon occlusion break. Prior art air vented cassettes allow ambient air to enter the aspiration line, however, venting air into the aspiration line changes the fluidic performance of the aspiration system by greatly increasing aspiration path compliance. Increased compliance can significantly increase the magnitude of occlusion break surge and also negatively affect system responsiveness. Liquid venting systems allow irrigation fluid to bleed into the aspiration line, thereby reducing any impact on the fluidic performance of the aspiration system. When higher aspiration vacuums are used, cassettes that vent the aspiration line to the irrigation line can cause high pressure surges in the irrigation line. Other systems provide a separate source of irrigation fluid to vent the aspiration line, requiring the use of two irrigation fluid sources and increasing the cost and complexity of the system.